As is well known to those skilled in the art, switching regulation circuits, specifically step-up ones, have applications in many communications apparatuses, such as cellular telephones, and similar battery-operated portable devices.
Such regulators comprise a final output stage which is ideally equipped with an LC filter, this term implying the provision of an Inductor-Capacitor circuit. However, due to the presence of parasitic resistances, the LC filter is practically bound to be an RLC filter, i.e., a Resistor-Inductor-Capacitor type of filter.
FIG. 1 of the accompanying drawings shows schematically a final output stage of a prior art regulator.
This output stage incorporates an RLC filter and has input IN and output OUT terminals with a parasitic resistance Rl and an inductance L in series therebetween.
Connected between the output terminal OUT and a ground GND is a capacitor C having a parasitic resistance Rc in series therewith.
More particularly, the resistance Rl represents the parasitic series resistance of the inductance L, while the resistance Rc represents the parasitic series resistance of the capacitor C.
The RLC circuit of FIG. 1 can be defined by the following equation: ##EQU1## where, Vin is a supply voltage applied to the input terminal IN of the output stage.
Assuming a supply voltage Vin with a waveform ramp from 0V to 4V through a 100 .mu.s time period, for example, the current transient at the RLC filter will be the waveform shown in FIG. 2A. Also, FIG. 2B shows the voltage trend across the capacitor C and parisitic resitance Rc (Vout), corresponding to the capacitor C being charged according to the transient of FIG. 2A.
Since the capacitor C is initially in a discharged condition, i.e., the voltage drop Vc across it is 0V, the current on the RLC filter will be only limited by the inductance and the parasitic resistances, with a damping value .xi. being given by the following relationship: ##EQU2##
In essence, there occurs a pronounced current pulse which subsides as soon as the capacitor C begins to be charged. This is indeed a current peak which can harm the inductor.
To prevent possible damage, more expensive inductors, capable of withstanding high current peaks, must be used.